Specific binding of the activator Mga to promoter sequences of the emm and scpA genes in the group A streptococcus

Association of CovRS two-component regulatory system with NADase induction by Clindamycin treatment in Streptococcus pyogenes

The administration of high-dose clindamycin (CLI) along with penicillin is recommended for the treatment of streptococcal toxic shock syndrome (STSS). However, CLI-resistant strains have been identified worldwide. Firstly, in this study, some CLI-resistant strains showed increased extracellular activities of the NAD- glycohydrolase (NADase) exotoxin after CLI treatment. This result supported our previous conclusion that not only CLI-susceptible but also CLI-resistant S. pyogenes strains show the CLI-dependent NADase induction. Secondary, using the 13 types of two- component-sensor knockout strains derived from a CLI-susceptible strain 1529 that has the CLI-dependent NADase induction phenotype, we investigated the mechanism of action. Among the knockout strains, only 1529ΔcovS lost the phenotype. In addition, 1529ΔspeB, 1529Δmga, and 1529Δrgg retained the CLI-dependent NADase induction phenotype. These results suggest that CovS is related to the phenotype in SpeB independent manner.

A novel RofA-family transcriptional regulator, GadR, controls the development of acid resistance in Listeria monocytogenes

IMPORTANCE

The ability to survive the acidic conditions found in the stomach is crucial for the food-borne pathogen Listeria monocytogenes to gain access to the mammalian gastrointestinal tract. Little is currently known about how acid resistance is regulated in this pathogen and why this trait is highly variable between strains. Here, we used comparative genomics to identify a novel RofA-family transcriptional regulator, GadR, that controls the development of acid resistance. The RofA family of regulators was previously found only in a small group of bacterial pathogens, including streptococci, where they regulate virulence properties. We show that gadR encodes the dominant regulator of acid resistance in L. monocytogenes and that its sequence variability accounts for previously observed differences between strains in this trait. Together, these findings significantly advance our understanding of how this important pathogen copes with acid stress and suggest a potential molecular target to aid its control in the food chain.

Slipped-strand mispairing within a polycytidine tract in transcriptional regulator mga leads to M protein phase variation and Mga length polymorphism in Group A Streptococcus

The M protein, a major virulence factor of Group A Streptococcus (GAS), is regulated by the multigene regulator Mga. An unexplained phenomena frequently occurring with in vitro genetic manipulation or culturing of M1T1 GAS strains is the loss of M protein production. This study was aimed at elucidating the basis for the loss of M protein production. The majority of M protein-negative (M^-) variants had one C deletion at a tract of 8 cytidines starting at base 1,571 of the M1 mga gene, which is designated as c.1571C[8]. The C deletion led to a c.1571C[7] mga variant that has an open reading frame shift and encodes a Mga-M protein fusion protein. Transformation with a plasmid containing wild-type mga restored the production of the M protein in the c.1571C[7] mga variant. Isolates producing M protein (M⁺) were recovered following growth of the c.1571C[7] M protein-negative variant subcutaneously in mice. The majority of the recovered isolates with reestablished M protein production had reverted back from c.1571C[7] to c.1571C[8] tract and some M⁺ isolates lost another C in the c.1571C[7] tract, leading to a c.1571C[6] variant that encodes a functional Mga with 13 extra amino acid residues at the C-terminus compared with wild-type Mga. The nonfunctional c.1571C[7] and functional c.1571C[6] variants are present in M1, M12, M14, and M23 strains in NCBI genome databases, and a G-to-A nonsense mutation at base 1,657 of M12 c.1574C[7] mga leads to a functional c.1574C[7]/1657A mga variant and is common in clinical M12 isolates. The numbers of the C repeats in this polycytidine tract and the polymorphism at base 1,657 lead to polymorphism in the size of Mga among clinical isolates. These findings demonstrate the slipped-strand mispairing within the c.1574C[8] tract of mga as a reversible switch controlling M protein production phase variation in multiple GAS common M types.

PRD-Containing Virulence Regulators (PCVRs) in Pathogenic Bacteria

Bacterial pathogens rely on a complex network of regulatory proteins to adapt to hostile and nutrient-limiting host environments. The phosphoenolpyruvate phosphotransferase system (PTS) is a conserved pathway in bacteria that couples transport of sugars with phosphorylation to monitor host carbohydrate availability. A family of structurally homologous PTS-regulatory-domain-containing virulence regulators (PCVRs) has been recognized in divergent bacterial pathogens, including Streptococcus pyogenes Mga and Bacillus anthracis AtxA. These paradigm PCVRs undergo phosphorylation, potentially via the PTS, which impacts their dimerization and their activity. Recent work with predicted PCVRs from Streptococcus pneumoniae (MgaSpn) and Enterococcus faecalis (MafR) suggest they interact with DNA like nucleoid-associating proteins. Yet, Mga binds to promoter sequences as a homo-dimeric transcription factor, suggesting a bi-modal interaction with DNA. High-resolution crystal structures of 3 PCVRs have validated the domain structure, but also raised additional questions such as how ubiquitous are PCVRs, is PTS-mediated histidine phosphorylation via potential PCVRs widespread, do specific sugars signal through PCVRs, and do PCVRs interact with DNA both as transcription factors and nucleoid-associating proteins? Here, we will review known and putative PCVRs based on key domain and functional characteristics and consider their roles as both transcription factors and possibly chromatin-structuring proteins.

Streptococcus pyogenes TrxSR Two-Component System Regulates Biofilm Production in Acidic Environments

Bacteria form biofilms for their protection against environmental stress and produce virulence factors within the biofilm. Biofilm formation in acidified environments is regulated by a two-component system, as shown by studies on isogenic mutants of the sensor protein of the two-component regulatory system in Streptococcus pyogenes. In this study, we found that the LiaS histidine kinase sensor mediates biofilm production and pilus expression in an acidified environment through glucose fermentation. The liaS isogenic mutant produced biofilms in a culture acidified by hydrochloric acid but not glucose, suggesting that the acidified environment is sensed by another protein. In addition, the trxS isogenic mutant could not produce biofilms or activate the mga promoter in an acidified environment. Mass spectrometry analysis showed that TrxS regulates M protein, consistent with the transcriptional regulation of emm, which encodes M protein. Our results demonstrate that biofilm production during environmental acidification is directly under the control of TrxS.

Application of the random forest algorithm to Streptococcus pyogenes response regulator allele variation: from machine learning to evolutionary models

Group A Streptococcus (GAS) is a globally significant bacterial pathogen. The GAS genotyping gold standard characterises the nucleotide variation of emm, which encodes a surface-exposed protein that is recombinogenic and under immune-based selection pressure. Within a supervised learning methodology, we tested three random forest (RF) algorithms (Guided, Ordinary, and Regularized) and 53 GAS response regulator (RR) allele types to infer six genomic traits (emm-type, emm-subtype, tissue and country of sample, clinical outcomes, and isolate invasiveness). The Guided, Ordinary, and Regularized RF classifiers inferred the emm-type with accuracies of 96.7%, 95.7%, and 95.2%, using ten, three, and four RR alleles in the feature set, respectively. Notably, we inferred the emm-type with 93.7% accuracy using only mga2 and lrp. We demonstrated a utility for inferring emm-subtype (89.9%), country (88.6%), invasiveness (84.7%), but not clinical (56.9%), or tissue (56.4%), which is consistent with the complexity of GAS pathophysiology. We identified a novel cell wall-spanning domain (SF5), and proposed evolutionary pathways depicting the 'contrariwise' and 'likewise' chimeric deletion-fusion of emm and enn. We identified an intermediate strain, which provides evidence of the time-dependent excision of mga regulon genes. Overall, our workflow advances the understanding of the GAS mga regulon and its plasticity.

Identification of two abundant Aerococcus urinae cell wall-anchored proteins

Aerococcus urinae is an emerging pathogen that causes urinary tract infections, bacteremia and infective endocarditis. The mechanisms through which A. urinae cause infection are largely unknown. The aims of this study were to describe the surface proteome of A. urinae and to analyse A. urinae genomes in search for genes encoding surface proteins. Two proteins, denoted Aerococcal surface protein (Asp) 1 and 2, were through the use of mass spectrometry based proteomics found to quantitatively dominate the aerococcal surface. The presence of these proteins on the surface was also shown using ELISA with serum from rabbits immunized with the recombinant Asp. These proteins had a signal sequence in the amino-terminal end and a cell wall-sorting region in the carboxy-terminal end, which contained an LPATG-motif, a hydrophobic domain and a positively charged tail. Twenty-three additional A. urinae genomes were sequenced using Illumina HiSeq technology. Six different variants of asp genes were found (denoted asp1-6). All isolates had either one or two of these asp-genes located in a conserved locus, designated Locus encoding Aerococcal Surface Proteins (LASP). The 25 genomes had in median 13 genes encoding LPXTG-proteins (range 6-24). For other Gram-positive bacteria, cell wall-anchored surface proteins with an LPXTG-motif play a key role for virulence. Thus, it will be of great interest to explore the function of the Asp proteins of A. urinae to establish a better understanding of the molecular mechanisms by which A. urinae cause disease.

Transcriptional activation by MafR, a global regulator of Enterococcus faecalis

Proteins that act as global transcriptional regulators play key roles in bacterial adaptation to new niches. These proteins recognize multiple DNA sites across the bacterial genome by different mechanisms. Enterococcus faecalis is able to survive in various niches of the human host, either as a commensal or as a leading cause of serious infections. Nonetheless, the regulatory pathways involved in its adaptive responses remain poorly understood. We reported previously that the MafR protein of E. faecalis causes genome-wide changes in the transcriptome. Here we demonstrate that MafR functions as a transcription activator. In vivo, MafR increased the activity of the P12294 and P11486 promoters and also the transcription levels of the two genes controlled by those promoters. These genes are predicted to encode a calcium-transporting P-type ATPase and a QueT transporter family protein, respectively. Thus, MafR could have a regulatory role in calcium homeostasis and queuosine synthesis. Furthermore, MafR recognized in vitro specific DNA sites that overlap the −35 element of each target promoter. The MafR binding sites exhibit a low sequence identity, suggesting that MafR uses a shape readout mechanism to achieve DNA-binding specificity.

Upstream sequence-dependent suppression and AtxA-dependent activation of protective antigens in Bacillus anthracis

The anthrax toxin is a virulence factor produced by the bacterium Bacillus anthracis. Transcription of anthrax toxin genes is controlled by the transcription factor AtxA. Thus, AtxA is thought to be a key factor for the pathogenicity of B. anthracis. Despite its important role in B. anthracis infection, the molecular mechanism by which AtxA controls expression of anthrax toxin remains unclear. This study aimed to characterize the molecular mechanism of AtxA-mediated regulation of protective antigen (PA), a component of anthrax toxin encoded by the pagA gene. First, the interaction between the upstream region of pagA and AtxA was evaluated in vivo by constructing a transcriptional fusion of the upstream region with an auxotrophic marker. The results showed that (i) the upstream region of pagA suppressed transcription of the downstream gene and (ii) AtxA recovered suppressed transcription. Second, in vitro analysis using a gel mobility shift assay was performed to evaluate binding specificity of the AtxA–DNA interaction. The result showed sequence-independent binding of AtxA to DNA. Taken together, our findings suggest that the expression of PA was suppressed by the upstream region of pagA and that an interaction of AtxA and the upstream region releases the suppression.

Contributions of different modules of the plasminogen-binding Streptococcus pyogenes M-protein that mediate its functional dimerization

Group A Streptococcus pyogenes (GAS) is a causative agent of pharyngeal and dermal infections in humans. A major virulence determinant of GAS is its dimeric signature fibrillar M-protein (M-Prt), which is evolutionarily designed in modules, ranging from a hypervariable extracellular N-terminal region to a progressively more highly conserved C-terminus that is covalently anchored to the cell wall. Of the >250 GAS isolates classified, only the subset of skin-trophic Pattern D strains expresses a specific serotype of M-Prt, PAM, that directly binds to host human plasminogen (hPg) via its extracellular NH2-terminal variable A-domain region. This interaction allows these GAS strains to accumulate components of the host fibrinolytic system on their surfaces to serve extracellular functions. While structure-function studies have been accomplished on M-Prts from Pattern A-C GAS isolates with different direct ligand binding properties compared to PAM, much less is known regarding the structure-function relationships of PAM-type M-Prts, particularly their dimerization determinants. To examine these questions, PAMs from seven GAS strains with sequence variations in the NH2-terminal ligand binding domains, as well as truncated versions of PAM, were designed and studied. The results from bioinformatic and biophysical analyses show that the different domains of PAM are disparately engaged in dimerization. From these data, we propose an experimentally-based model for PAM secondary and quaternary structures that is highly dependent on the conserved helical C-terminal C-D-domains. In addition, while the N-terminal regions of PAMs are variable in sequence, the binding properties of hPg and its activated product, plasmin, to the A-domain, remain intact.

DNA-binding properties of MafR, a global regulator of Enterococcus faecalis

Global transcriptional regulators play key roles during bacterial adaptation to environmental fluctuations. Protein MafR from Enterococcus faecalis was shown to activate the transcription of many genes on a genome-wide scale. We proposed that MafR is a global regulator of the Mga/AtxA family. Here, we purified an untagged form of the MafR protein and found that it binds to linear double-stranded DNAs in a nonsequence-specific manner. Moreover, multiple MafR units (likely dimers) bind sequentially to the DNA molecule generating multimeric complexes. On DNAs that contain the promoter of the mafR gene, MafR recognizes a potentially curved DNA region. We discuss that a characteristic of the Mga/AtxA regulators might be their ability to recognize particular DNA shapes across the bacterial genomes.

Genome Analysis of Streptococcus pyogenes Associated with Pharyngitis and Skin Infections

Streptococcus pyogenes is a very important human pathogen, commonly associated with skin or throat infections but can also cause life-threatening situations including sepsis, streptococcal toxic shock syndrome, and necrotizing fasciitis. Various studies involving typing and molecular characterization of S. pyogenes have been published to date; however next-generation sequencing (NGS) studies provide a comprehensive collection of an organism’s genetic variation. In this study, the genomes of nine S. pyogenes isolates associated with pharyngitis and skin infection were sequenced and studied for the presence of virulence genes, resistance elements, prophages, genomic recombination, and other genomic features. Additionally, a comparative phylogenetic analysis of the isolates with global clones highlighted their possible evolutionary lineage and their site of infection. The genomes were found to also house a multitude of features including gene regulation systems, virulence factors and antimicrobial resistance mechanisms.

Crystal structure of Bacillus anthracis virulence regulator AtxA and effects of phosphorylated histidines on multimerization and activity

The Bacillus anthracis virulence regulator AtxA controls transcription of the anthrax toxin genes and capsule biosynthetic operon. AtxA activity is elevated during growth in media containing glucose and CO(2)/bicarbonate, and there is a positive correlation between the CO(2)/bicarbonate signal, AtxA activity and homomultimerization. AtxA activity is also affected by phosphorylation at specific histidines. We show that AtxA crystallizes as a dimer. Distinct folds associated with predicted DNA-binding domains (HTH1 and HTH2) and phosphoenolpyruvate: carbohydrate phosphotransferase system-regulated domains (PRD1 and PRD2) are apparent. We tested AtxA variants containing single and double phosphomimetic (His→Asp) and phosphoablative (His→Ala) amino acid changes for activity in B. anthracis cultures and for protein-protein interactions in cell lysates. Reduced activity of AtxA H199A, lack of multimerization and activity of AtxAH379D variants, and predicted structural changes associated with phosphorylation support a model for control of AtxA function. We propose that (i) in the AtxA dimer, phosphorylation of H199 in PRD1 affects HTH2 positioning, influencing DNA-binding; and (ii) phosphorylation of H379 in PRD2 disrupts dimer formation. The AtxA structure is the first reported high-resolution full-length structure of a PRD-containing regulator, and can serve as a model for proteins of this family, especially those that link virulence to bacterial metabolism.

A naturally occurring single amino acid replacement in multiple gene regulator of group A Streptococcus significantly increases virulence

Single-nucleotide polymorphisms (SNPs) are the most common source of genetic variation within a species; however, few investigations demonstrate how naturally occurring SNPs may increase strain virulence. We recently used group A Streptococcus as a model pathogen to study bacteria strain genotype-patient disease phenotype relationships. Whole-genome sequencing of approximately 800 serotype M59 group A Streptococcus strains, recovered during an outbreak of severe invasive infections across North America, identified a disproportionate number of SNPs in the gene encoding multiple gene regulator of group A Streptococcus (mga). Herein, we report results of studies designed to test the hypothesis that the most commonly occurring SNP, encoding a replacement of arginine for histidine at codon 201 of Mga (H201R), significantly increases virulence. Whole transcriptome analysis revealed that the H201R replacement significantly increased expression of mga and 54 other genes, including many proven virulence factors. Compared to the wild-type strain, a H201R isogenic mutant strain caused significantly larger skin lesions in mice. Serial quantitative bacterial culture and noninvasive magnetic resonance imaging also demonstrated that the isogenic H201R strain was significantly more virulent in a nonhuman primate model of joint infection. These findings show that the H201R replacement in Mga increases the virulence of M59 group A Streptococcus and provide new insight to how a naturally occurring SNP in bacteria contributes to human disease phenotypes.

Production of hyaluronic acid by mutant strains of group C Streptococcus

This study addresses the influence of upstream region sequence on the strength of has operon promoter in highly encapsulated S. equi subsp. zooepidemicus (SEZ). For this purpose, seven different strains were constructed. Each strain carries a point mutation in one of the following positions upstream of the has promoter: -43, -44, -49, and -50 bp. To facilitate measuring of the recombinant promoter relative strength, ß-glucuronidase gene was used as a reporter gene. Three mutations located in positions -49 and -50: AT, GT, and AG, positively impacted has promoter strength when compared to the wild type sequence GG. Conversely, two other mutations: TG and TT, exhibited a slight inhibitory effect. Further, three different strains carrying chromosomal mutations in the has promoter region were constructed. In two cases, the has operon is under the control of a stronger promoter and in the third strain the has operon is controlled by a weaker promoter. The laboratory fermenter scale cultivations confirmed the increase of hyaluronan yields for SEZPhasAG and SEZPhas2G, resulting 116 and 105 %, respectively. As expected, the yield of the hyaluronic acid of SEZPhas2B strain fell to 41 %.

Natural variation in the promoter of the gene encoding the Mga regulator alters host-pathogen interactions in group a Streptococcus carrier strains

Humans commonly carry pathogenic bacteria asymptomatically, but the molecular factors underlying microbial asymptomatic carriage are poorly understood. We previously reported that two epidemiologically unassociated serotype M3 group A Streptococcus (GAS) carrier strains had an identical 12-bp deletion in the promoter of the gene encoding Mga, a global positive gene regulator. Herein, we report on studies designed to test the hypothesis that the identified 12-bp deletion in the mga promoter alters GAS virulence, thereby potentially contributing to the asymptomatic carrier phenotype. Using allelic exchange, we introduced the variant promoter into a serotype M3 invasive strain and the wild-type promoter into an asymptomatic carrier strain. Compared to strains with the wild-type mga promoter, we discovered that strains containing the promoter with the 12-bp deletion produced significantly fewer mga and Mga-regulated gene transcripts. Consistent with decreased mga transcripts, strains containing the variant mga promoter were also significantly less virulent in in vivo and ex vivo models of GAS disease. Further, we provide evidence that the pleiotropic regulator protein CodY binds to the mga promoter and that the 12-bp deletion in the mga promoter reduces CodY-mediated mga transcription. We conclude that the naturally occurring 12-bp deletion in the mga promoter significantly alters the pathogen-host interaction of these asymptomatic carrier strains. Our findings provide new insight into the molecular basis of the carrier state of an important human pathogen.

The pneumococcal MgaSpn virulence transcriptional regulator generates multimeric complexes on linear double-stranded DNA

The MgaSpn transcriptional regulator contributes to the virulence of Streptococcus pneumoniae. It is thought to be a member of the Mga/AtxA family of global regulators. MgaSpn was shown to activate in vivo the P1623B promoter, which is divergent from the promoter (Pmga) of its own gene. This activation required a 70-bp region (PB activation region) located between both promoters. In this work, we purified an untagged form of the MgaSpn protein, which formed dimers in solution. By gel retardation and footprinting assays, we analysed the binding of MgaSpn to linear double-stranded DNAs. MgaSpn interacted with the PB activation region when it was placed at internal position on the DNA. However, when it was positioned at one DNA end, MgaSpn recognized preferentially the Pmga promoter placed at internal position. In both cases, and on binding to the primary site, MgaSpn spread along the adjacent DNA regions generating multimeric protein–DNA complexes. When both MgaSpn-binding sites were located at internal positions on longer DNAs, electron microscopy experiments demonstrated that the PB activation region was the preferred target. DNA molecules totally or partially covered by MgaSpn were also visualized. Our results suggest that MgaSpn might recognize particular DNA conformations to achieve DNA-binding specificity.

PTS phosphorylation of Mga modulates regulon expression and virulence in the group A streptococcus

The ability of a bacterial pathogen to monitor available carbon sources in host tissues provides a clear fitness advantage. In the group A streptococcus (GAS), the virulence regulator Mga contains homology to phosphotransferase system (PTS) regulatory domains (PRDs) found in sugar operon regulators. Here we show that Mga was phosphorylated in vitro by the PTS components EI/HPr at conserved PRD histidines. A ΔptsI (EI-deficient) GAS mutant exhibited decreased Mga activity. However, PTS-mediated phosphorylation inhibited Mga-dependent transcription of emm in vitro. Using alanine (unphosphorylated) and aspartate (phosphomimetic) mutations of PRD histidines, we establish that a doubly phosphorylated PRD1 phosphomimetic (D/DMga4) is completely inactive in vivo, shutting down expression of the Mga regulon. Although D/DMga4 is still able to bind DNA in vitro, homo-multimerization of Mga is disrupted and the protein is unable to activate transcription. PTS-mediated regulation of Mga activity appears to be important for pathogenesis, as bacteria expressing either non-phosphorylated (A/A) or phosphomimetic (D/D) PRD1 Mga mutants were attenuated in a model of GAS invasive skin disease. Thus, PTS-mediated phosphorylation of Mga may allow the bacteria to modulate virulence gene expression in response to carbohydrate status. Furthermore, PRD-containing virulence regulators (PCVRs) appear to be widespread in Gram-positive pathogens.

A natural inactivating mutation in the CovS component of the CovRS regulatory operon in a pattern D Streptococcal pyogenes strain influences virulence-associated genes

A skin-tropic invasive group A Streptococcus pyogenes (GAS) strain, AP53, contains a natural inactivating mutation in the covS gene (covS(M)) of the two-component responder (CovR)/sensor (CovS) gene regulatory system. The effects of this mutation on specific GAS virulence determinants have been assessed, with emphasis on expression of the extracellular protease, streptococcal pyrogenic exotoxin B (SpeB), capsular hyaluronic acid, and proteins that allow host plasmin assembly on the bacterial surface, viz. a high affinity plasminogen (Pg)/plasmin receptor, Pg-binding group A streptococcal M protein (PAM), and the human Pg activator streptokinase. To further illuminate mechanisms of the functioning of CovRS in the virulence of AP53, two AP53 isogenic strains were generated, one in which the natural covS(M) gene was mutated to WT-covS (AP53/covS(WT)) and a strain that contained an inactivated covR gene (AP53/ΔcovR). Two additional strains that do not contain PAM, viz. WT-NS931 and NS931/covS(M), were also employed. SpeB was not measurably expressed in strains containing covR(WT)/covS(M), whereas in strains with natural or engineered covR(WT)/covS(WT), SpeB expression was highly up-regulated. Alternatively, capsule synthesis via the hasABC operon was enhanced in strain AP53/covS(M), whereas streptokinase expression was only slightly affected by the covS inactivation. PAM expression was not substantially influenced by the covS mutation, suggesting that covRS had minimal effects on the mga regulon that controls PAM expression. These results demonstrate that a covS inactivation results in virulence gene alterations and also suggest that the CovR phosphorylation needed for gene up- or down-regulation can occur by alternative pathways to CovS kinase.

Regulation of inhibition of neutrophil infiltration by the two-component regulatory system CovRS in subcutaneous murine infection with group A streptococcus

Hypervirulent invasive group A streptococcus (GAS) isolates inhibit neutrophil infiltration more than pharyngitis isolates do, and the molecular basis of this difference is not well understood. This study was designed to first determine whether natural null mutation of the two-component regulatory system CovRS is responsible for the enhancement of the inhibition of neutrophil recruitment seen in hypervirulent GAS. Next, we examined the role of CovRS-regulated interleukin-8/CXC chemokine peptidase (SpyCEP), C5a peptidase (ScpA), and platelet-activating factor acetylhydrolase (SsE) in the enhanced innate immune evasion. Invasive isolate MGAS5005 induces less neutrophil infiltration and produced a greater lesion area than pharyngitis isolate MGAS2221 in subcutaneous infections of mice. It is known that MGAS5005, but not MGAS2221, has a natural 1-bp deletion in the covS gene. Replacement of covS(Δ1bp) in MGAS5005 with wild-type covS resulted in the MGAS2221 phenotype. Deletion of covS from MGAS2221 resulted in the MGAS5005 phenotype. Tests of single, double, and triple deletion mutants of the MGAS5005 sse, spyCEP, and scpA genes found that SsE plays a more important role than SpyCEP and ScpA in the inhibition of neutrophil recruitment and that SsE, SpyCEP, and ScpA do not have synergistic effects on innate immune evasion by MGAS5005. Deletion of sse, but not spyCEP or scpA, of MGAS2221 enhances neutrophil recruitment. Thus, covS null mutations can cause substantial inhibition of neutrophil recruitment by enhancing the expression of the chemoattractant-degrading virulence factors, and SsE, but not SpyCEP or ScpA, is required for CovRS-regulated GAS inhibition of neutrophil infiltration.

RivR is a negative regulator of virulence factor expression in group A Streptococcus

The bacterial pathogen group A Streptococcus (GAS) causes human diseases ranging from self-limiting pharyngitis (also known as strep throat) to severely invasive necrotizing fasciitis (also known as the flesh-eating syndrome). To control virulence factor expression, GAS utilizes both protein- and RNA-based mechanisms of regulation. Here we report that the transcription factor RivR (RofA-like protein IV) negatively regulates the abundance of mRNAs encoding the hyaluronic acid capsule biosynthesis proteins (hasABC; ∼7-fold) and the protein G-related α(2)-macroglobulin-binding protein (grab; ∼29-fold). Our data differ significantly from those of a previous study of the RivR regulon. Given that grab and hasABC are also negatively regulated by the two-component system CovR/S (control of virulence), we tested whether RivR functions through CovR/S. A comparison of riv and cov single and double mutant strains showed that RivR requires CovR activity for grab and hasABC regulation. Analysis of the upstream region of rivR identified a novel promoter the deletion of which reduced rivR mRNA abundance by 70%. A rivR mutant strain had a reduced ability to adhere to human keratinocytes relative to that of the parental and complemented strains, a phenotype that was abolished upon GAS pretreatment with hyaluronidase, highlighting the importance of capsule regulation by RivR during colonization. The rivR mutant strain was also attenuated for virulence in a murine model of bacteremia infection. Thus, we identify RivR as an important regulator of GAS virulence and provide new insight into the regulatory networks controlling virulence factor production in this pathogen.

The conserved surface M-protein SiMA of Streptococcus iniae is not effective as a cross-protective vaccine against differing capsular serotypes in farmed fish

Streptococcus iniae causes invasive infections in fresh and saltwater fish and occasional zoonoses. Vaccination against S. iniae is complicated by serotypic variation determined by capsular polysaccharide. A potential target for serologically cross-protective vaccines is the M-like protein SiMA, an essential virulence factor in S. iniae that is highly conserved amongst virulent strains. The present study determined how SiMA is regulated and investigated potential as a cross-protective vaccine for fish. Electrophoretic mobility shift suggested that SiMA is regulated by the multigene regulator Mgx via a binding site in the -35 region of the simA promoter. Moreover, expression of simA and mgx was highly correlated, with the highest level of simA and mgx expression during exponential growth under iron limitation (20-fold increase in relative expression compared to growth in Todd-Hewitt broth). Based on these results, a vaccination and challenge experiment was conducted in barramundi (Lates calcarifer) to determine whether SiMA is protective against S. iniae infection and cross-protective against a different capsular serotype. The challenge resulted in 60% mortality in control fish. Formalin-killed bacterins prepared from the challenge strain resulted in 100% protection, whereas bacterins prepared from a serotypically heterologous strain resulted in significantly reduced protection, even when culture conditions were manipulated to optimise SiMA expression. Moreover, recombinant SiMA protein was not protective against the challenge strain in spite of eliciting specific antibody response in vaccinated fish. Specific antibody did not increase oxidative activity or phagocytosis by barramundi macrophages. Indeed incubating S. iniae with antisera significantly reduced phagocytosis. Lack of specific-antibody mediated opsonisation in spite of 100% protection against challenge with the homologous vaccine suggests that other immune parameters result in protection of challenged fish.

Characterization of the Group A Streptococcus Mga virulence regulator reveals a role for the C-terminal region in oligomerization and transcriptional activation

The Group A Streptococcus (GAS) is a strict human pathogen that causes a broad spectrum of illnesses. One of the key regulators of virulence in GAS is the transcriptional activator Mga, which co-ordinates the early stages of infection. Although the targets of Mga have been well characterized, basic biochemical analyses have been limited due to difficulties in obtaining purified protein. In this study, high-level purification of soluble Mga was achieved, enabling the first detailed characterization of the protein. Fluorescence titrations coupled with filter-binding assays indicate that Mga binds cognate DNA with nanomolar affinity. Gel filtration analyses, analytical ultracentrifugation and co-immunoprecipitation experiments demonstrate that Mga forms oligomers in solution.Moreover, the ability of the protein to oligomerize in solution was found to correlate with transcriptional activation; DNA binding appears to be necessary but insufficient for full activity. Truncation analyses reveal that the uncharacterized C-terminal region of Mga, possessing similarity to phosphotransferase system EIIB proteins, plays a critical role in oligomerization and in vivo activity. Mga from a divergent serotype was found to behave similarly, suggesting that this study describes a general mechanism for Mga regulation of target virulence genes within GAS and provides insight into related regulators in other Gram-positive pathogens.

Nucleotides critical for the interaction of the Streptococcus pyogenes Mga virulence regulator with Mga-regulated promoter sequences

The Mga regulator of Streptococcus pyogenes directly activates the transcription of a core regulon that encodes virulence factors such as M protein (emm), C5a peptidase (scpA), and streptococcal inhibitor of complement (sic) by directly binding to a 45-bp binding site as determined by an electrophoretic mobility shift assay (EMSA) and DNase I protection. However, by comparing the nucleotide sequences of all established Mga binding sites, we found that they exhibit only 13.4% identity with no discernible symmetry. To determine the core nucleotides involved in functional Mga-DNA interactions, the M1T1 Pemm1 binding site was altered and screened for nucleotides important for DNA binding in vitro and for transcriptional activation using a plasmid-based luciferase reporter in vivo. Following this analysis, 34 nucleotides within the Pemm1 binding site that had an effect on Mga binding, Mga-dependent transcriptional activation, or both were identified. Of these critical nucleotides, guanines and cytosines within the major groove were disproportionately identified clustered at the 5' and 3' ends of the binding site and with runs of nonessential adenines between the critical nucleotides. On the basis of these results, a Pemm1 minimal binding site of 35 bp bound Mga at a level comparable to the level of binding of the larger 45-bp site. Comparison of Pemm with directed mutagenesis performed in the M1T1 Mga-regulated PscpA and Psic promoters, as well as methylation interference analysis of PscpA, establish that Mga binds to DNA in a promoter-specific manner.

Growth phase-dependent modulation of Rgg binding specificity in Streptococcus pyogenes

Streptococcus pyogenes Rgg is a transcriptional regulator that interacts with the cofactor LacD.1 to control growth phase-dependent expression of genes, including speB, which encodes a secreted cysteine protease. LacD.1 is thought to interact with Rgg when glycolytic intermediates are abundant in a manner that prevents Rgg-mediated activation of speB expression via binding to the promoter region. When the intermediates diminish, LacD.1 dissociates from Rgg and binds to the speB promoter to activate expression. The purpose of this study was to determine if Rgg bound to chromatin during the exponential phase of growth and, if so, to identify the binding sites. Rgg bound to 62 chromosomal sites, as determined by chromatin immunoprecipitation coupled with DNA microarrays. Thirty-eight were within noncoding DNA, including sites upstream of the genes encoding the M protein (M49), serum opacity factor (SOF), fibronectin-binding protein (SfbX49), and a prophage-encoded superantigen, SpeH. Each of these sites contained a promoter that was regulated by Rgg, as determined with transcriptional fusion assays. Purified Rgg also bound to the promoter regions of emm49, sof, and sfbX49 in vitro. Results obtained with a lacD.1 mutant showed that both LacD.1 and Rgg were necessary for the repression of emm49, sof, sfbX49, and speH expression. Overall, the results indicated that the DNA binding specificity of Rgg is responsive to environmental changes in a LacD.1-dependent manner and that Rgg and LacD.1 directly control virulence gene expression in the exponential phase of growth.

CcpA-dependent carbohydrate catabolite repression regulates galactose metabolism in Streptococcus oligofermentans

Streptococcus oligofermentans is an oral commensal that inhibits the growth of the caries pathogen Streptococcus mutans by producing copious amounts of H(2)O(2) and that grows faster than S. mutans on galactose. In this study, we identified a novel eight-gene galactose (gal) operon in S. oligofermentans that was comprised of lacABCD, lacX, and three genes encoding a galactose-specific transporter. Disruption of lacA caused more growth reduction on galactose than mutation of galK, a gene in the Leloir pathway, indicating that the principal role of this operon is in galactose metabolism. Diauxic growth was observed in cultures containing glucose and galactose, and a luciferase reporter fusion to the putative gal promoter demonstrated 12-fold repression of the operon expression by glucose but was induced by galactose, suggesting a carbon catabolite repression (CCR) control in galactose utilization. Interestingly, none of the single-gene mutations in the well-known CCR regulators ccpA and manL affected diauxic growth, although the operon expression was upregulated in these mutants in glucose. A double mutation of ccpA and manL eliminated glucose repression of galactose utilization, suggesting that these genes have parallel functions in regulating gal operon expression and mediating CCR. Electrophoretic mobility shift assays demonstrated binding of CcpA to the putative catabolite response element motif in the promoter regions of the gal operon and manL, suggesting that CcpA regulates CCR through direct regulation of the transcription of the gal operon and manL. This provides the first example of oral streptococci using two parallel CcpA-dependent CCR pathways in controlling carbohydrate metabolism.

Structure of the has operon promoter and the effect of mutations on the has promoter strength in Streptococcus equi subsp. zooepidemicus

The purpose of this study is to determine the effect of the corresponding nucleotides from Streptococcus pyogenes on the has promoter strength in highly encapsulated strain S. equi subsp. zooepidemicus (SEZ) and detect an empowering mutations in SEZ. Eight different strains of SEZ carrying nucleotide mutations in the -73 to -38 region upstream of the has promoter were constructed. The significant activity decrease to 36-1% was observed after the introduction of mutations in the promoter region from -44 to -38 site. The exception was observed in mutation in -49 site when no significant decrease was observed. When nucleotides TTT were used in positions -73 the promoter became weaker, whereas no significant effect was observed after using nucleotides CCC (96%). Unfortunately, introduction of these mutations into chromosome SEZ has no empowering effect. Six strains, which carried nucleotide sequences of different lengths upstream from the transcription start of hasA promoter, were constructed to determine the minimum upstream region required for the maximum transcription efficiency of the has operon. No change of the activity of the has promoter constructs containing as few as 101 nucleotides upstream from the transcription start point was observed.

Common regulators of virulence in streptococci

Streptococcal species are a diverse group of bacteria which can be found in animals and humans. Their interactions with host organisms can vary from commensal to pathogenic. Many of the pathogenic species are causative agents of severe, invasive infections in their hosts, accounting for a high burden of morbidity and mortality, associated with high economic costs in industry and health care. Among them, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae, and Streptococcus suis are discussed here. An environmentally stimulated and tightly controlled expression of their virulence factors is of utmost importance for their pathogenic potential. Thus, the most universal and widespread regulators from the classes of stand-alone transcriptional regulators, two-component signal transduction systems (TCS), eukaryotic-like serine/threonine kinases, and small noncoding RNAs are the topic of this chapter. The regulatory levels are reviewed with respect to function, activity, and their role in pathogenesis. Understanding of and interfering with transcriptional regulation mechanisms and networks is a promising basis for the development of novel anti-infective therapies.

Bacillus anthracis virulence regulator AtxA: oligomeric state, function and CO(2) -signalling

AtxA, a unique regulatory protein of unknown molecular function, positively controls expression of the major virulence genes of Bacillus anthracis. The 475 amino acid sequence of AtxA reveals DNA binding motifs and regions similar to proteins associated with the phosphoenolpyruvate: carbohydrate phosphotransferase system (PTS). We used strains producing native and functional epitope-tagged AtxA proteins to examine protein-protein interactions in cell lysates and in solutions of purified protein. Co-affinity purification, non-denaturing polyacrylamide gel electrophoresis and bis(maleimido)hexane (BMH) cross-linking experiments revealed AtxA homo-multimers. Dimers were the most abundant species. BMH cross-links available cysteines within 13 Å. To localize interaction sites, six AtxA mutants containing distinct Cys→Ser substitutions were tested for multimerization and cross-linking. All mutants multimerized, but one mutation, C402S, prevented cross-linking. Thus, BMH uses C402 to make the inter-molecular bond between AtxA proteins, but C402 is not required for protein-protein interaction. C402 is in a region bearing amino acid similarity to Enzyme IIB proteins of the PTS. The AtxA EIIB motif may function in protein oligomerization. Finally, cultures grown with elevated CO(2) /bicarbonate exhibited increased AtxA dimer/monomer ratios and increased AtxA activity, relative to cultures grown without added CO(2) /bicarbonate, suggesting that this host-associated signal enhances AtxA function by shifting the dimer/monomer equilibrium towards the dimeric state.

Analysis of two-component sensor proteins involved in the response to acid stimuli in Streptococcus pyogenes

The virulence of Streptococcus pyogenes depends on proteins that are produced by this bacterium. The production of virulence proteins depends on environmental factors, and two-component regulatory systems are considered to be involved in sensing these factors. One of the environmental factors is acid stimuli. We established knockout strains in all speculated two-component regulatory sensor proteins of the M1 clinical strain of S. pyogenes and examined their relevance to acid stimuli. The parental strain and its derived knockout strains were cultured in a medium adjusted to pH 7.6 or 6.0, and their growth in broth was compared. The spy1622 sensor knockout strain showed significant growth reduction compared with the parental strain in broth at pH 6.0, suggesting that the Spy1622 two-component sensor protein is involved in sensing acid stimuli. To further examine the role of the Spy1622 two-component sensor protein in virulence, blood bactericidal assays and mouse infection model experiments were performed. We found that the spy1622 knockout strain was less virulent than the parental strain, which suggests that the Spy1622 two-component sensor protein could play an important role in virulence.

Streptococcus iniae M-like protein contributes to virulence in fish and is a target for live attenuated vaccine development

Background

Streptococcus iniae is a significant pathogen in finfish aquaculture, though knowledge of virulence determinants is lacking. Through pyrosequencing of the S. iniae genome we have identified two gene homologues to classical surface-anchored streptococcal virulence factors: M-like protein (simA) and C5a peptidase (scpI).

Methodology/Principal Findings

S. iniae possesses a Mga-like locus containing simA and a divergently transcribed putative mga-like regulatory gene, mgx. In contrast to the Mga locus of group A Streptococcus (GAS, S. pyogenes), scpI is located distally in the chromosome. Comparative sequence analysis of the Mgx locus revealed only one significant variant, a strain with an insertion frameshift mutation in simA and a deletion mutation in a region downstream of mgx, generating an ORF which may encode a second putative mga-like gene, mgx2. Allelic exchange mutagenesis of simA and scpI was employed to investigate the potential role of these genes in S. iniae virulence. Our hybrid striped bass (HSB) and zebrafish models of infection revealed that M-like protein contributes significantly to S. iniae pathogenesis whereas C5a peptidase-like protein does not. Further, in vitro cell-based analyses indicate that SiMA, like other M family proteins, contributes to cellular adherence and invasion and provides resistance to phagocytic killing. Attenuation in our virulence models was also observed in the S. iniae isolate possessing a natural simA mutation. Vaccination of HSB with the ΔsimA mutant provided 100% protection against subsequent challenge with a lethal dose of wild-type (WT) S. iniae after 1,400 degree days, and shows promise as a target for live attenuated vaccine development.

Conclusions/Significance

Analysis of M-like protein and C5a peptidase through allelic replacement revealed that M-like protein plays a significant role in S. iniae virulence, and the Mga-like locus, which may regulate expression of this gene, has an unusual arrangement. The M-like protein mutant created in this research holds promise as live-attenuated vaccine.

Identification and molecular characterisation of a fibrinogen binding protein from Streptococcus iniae

Background

Binding of serum components by surface M-related proteins, encoded by the emm genes, in streptococci constitutes a major virulence factor in this important group of organisms. The present study demonstrates fibrinogen binding by S. iniae, a Lancefield non-typeable pathogen causing devastating fish losses in the aquaculture industry and an opportunistic pathogen of humans, and identifies the proteins involved and their encoding genes.

Results

Fibrinogen binding by S. iniae significantly reduced respiratory burst activity of barramundi peritoneal macrophages in primary cultures compared to BSA-treated or untreated controls, indicating a potentially important role for fibrinogen binding cell-surface proteins in avoiding phagocytic attack in fish. We describe a novel emm-like gene, simA, encoding a 57 kDa fibrinogen binding M-like protein in S. iniae. These SiM proteins and their corresponding tetrameric structures from some sequevar types (~230 kDa) bound fibrinogen in Western blots. simA was most closely related (32% identity) to the demA gene of S. dysgalactiae. Genome walking and sequencing determined the genetic organization of the simA region had similarities to the mgrC regulon in GCS and to S. uberis. Moreover, a putative multigene regulator, mgx was orientated in the opposite direction to the simA gene in common with S. uberis, but contrary to findings in GAS and GCS. In GAS, diversity among emm-genes and consequent diversity of their M-related proteins results in substantial antigenic variation. However, an extensive survey of S. iniae isolates from diverse geographic regions and hosts revealed only three variants of the gene, with one sequevar accounting for all but two of the 50 isolates analysed.

Conclusion

These proteins play a role in avoiding oxidative attack by phagocytic cells during infection of fish by S. iniae, but genetic diversity amongst these key surface proteins has not yet arisen. This lack of diversity coupled with a functional role in macrophage resistance suggests that these proteins may constitute important targets for future vaccines against S. iniae in fish.

Analysis of the Campylobacter jejuni FlgR response regulator suggests integration of diverse mechanisms to activate an NtrC-like protein

Flagellar motility in Campylobacter jejuni mediates optimal interactions with human or animal hosts. Sigma(54) and the FlgSR two-component system are necessary for the expression of many C. jejuni flagellar genes. The FlgR response regulator is homologous to the NtrC family of transcriptional activators. These regulators usually contain an N-terminal receiver domain, a central domain that interacts with sigma(54) and hydrolyzes ATP, and a DNA-binding C-terminal domain. Most often, phosphorylation of the receiver domain influences its inherent ability to either positively or negatively control the activity of the regulator. In this study, we performed genetic and biochemical analyses to understand how FlgR activity is controlled to culminate in the expression of sigma(54)-dependent flagellar genes. Our data suggest that the FlgR receiver domain has the capacity for both positive and negative regulation in controlling the activation of the protein. Analysis of the C-terminal domain of FlgR revealed that it lacks a DNA-binding motif and is not required for sigma(54)-dependent flagellar gene expression. Further analysis of FlgR lacking the C-terminal domain indicates that this protein is partially functional in the absence of the cognate sensor kinase, FlgS, but its activity is still dependent on the phosphorylated residue in the receiver domain, D51. We hypothesize that the C-terminal domain may not function to bind DNA but may ensure the specificity of the phosphorylation of FlgR by FlgS. Our results demonstrate that FlgR activation mechanisms are unusual among characterized NtrC-like proteins and emphasize that various means are utilized by the NtrC family of proteins to control the transcription of target genes.

RivR and the small RNA RivX: the missing links between the CovR regulatory cascade and the Mga regulon

The CovR/S two-component system regulates the transcription of many genes that are crucial for the virulence of Streptococcus pyogenes (group A Streptococcus, GAS). Previously, we demonstrated that one gene repressed directly by CovR is rivR, which encodes a member of the RofA-like family of transcriptional regulators. In this study, we deleted rivR and its downstream gene rivX in a DeltacovR background. Microarray analysis revealed that the products of the rivRX locus exert positive control over the transcription of members of the Mga regulon. Using mutational analysis, we established that rivX encodes a small regulatory RNA. We found that RivR enhances transcriptional activation by Mga in vivo and in vitro. An M1 DeltacovRDeltarivRX strain is attenuated for virulence in a murine model of invasive soft tissue infection and this attenuation is complemented by rivRX expressed from a plasmid, demonstrating the importance of the rivRX locus in pathogenesis. This study provides the first link between the CovR and Mga regulatory networks. By integrating the signals received through these two global regulators, GAS is able to select from its repertoire different combinations of specific virulence factors to express in response to a broad spectrum of environmental conditions.

The catabolite control protein CcpA binds to Pmga and influences expression of the virulence regulator Mga in the Group A streptococcus

Carbon catabolite repression (CCR) allows bacteria to alter metabolism in response to the availability of specific sugar sources, and increasing evidence suggests that CCR is involved in regulating virulence gene expression in many pathogens. A scan of the M1 SF370 group A streptococcus (GAS) genome using a Bacillus subtilis consensus identified a number of potential catabolite-responsive elements (cre) important for binding by the catabolite control protein A (CcpA), a mediator of CCR in gram-positive bacteria. Intriguingly, a putative cre was identified in the promoter region of mga upstream of its distal P1 start of transcription. Electrophoretic mobility shift assays showed that a His-CcpA fusion protein was capable of binding specifically to the cre in Pmga in vitro. Deletion analysis of Pmga using single-copy Pmga-gusA reporter strains found that Pmga P1 and its upstream cre were not required for normal autoregulated mga expression from Pmga P2 as long as Mga was produced from its native locus. In fact, the Pmga P1 region appeared to show a negative influence on Pmga P2 in these studies. However, deletion of the cre at the native Pmga resulted in a reduction of total mga transcripts as determined by real-time reverse transcription-PCR, supporting a role for CcpA in initial expression. Furthermore, normal transcriptional initiation from the Pmga P1 start site alone was dependent on the presence of the cre. Importantly, inactivation of ccpA in the M6 GAS strain JRS4 resulted in a reduction in Pmga expression and Mga protein levels in late-logarithmic-phase cell growth. These data support a role for CcpA in the early activation of the mga promoter and establish a link between CCR and Mga regulation in the GAS.

Defining the Mga regulon: comparative transcriptome analysis reveals both direct and indirect regulation by Mga in the group A streptococcus

The regulator Mga in the group A streptococcus (GAS) is known to directly activate several virulence genes important for colonization and immune evasion. Transcriptome analysis comparing two mga-1 serotypes (M1 SF370, M6 JRS4) and one mga-2 serotype (M4 GA40634) against their isogenic mga-inactivated strains uncovered a broader Mga regulon profile containing both activated and repressed genes with predicted functions primarily related to sugar metabolism. This was reflected in the altered abilities of M1 and M4 Mga mutants to grow in chemically defined media with a single sugar source compared with their wild-type counterparts. Although the M1 and M4 Mga profiles were similar, the M6 JRS4 was clearly distinct, even from other M6 strains. Real-time RT-PCR and Northern blots confirmed that established core Mga regulon genes directly activated by Mga (emm, scpA, sof, fba) exhibited the highest activation levels across all strains tested. Spy2036 encoding a cytosolic hypothetical protein was highly activated in all three serotypes and was called gene regulated by Mga (grm). Mga bound directly to Pgrm, which overlaps the Mga-regulated Psof in OF+ strains, suggesting that grm is part of the core Mga regulon and Mga is able to activate divergently transcribed genes from a single site. Furthermore, Mga activated speB when detectable in the wild-type strain, although direct binding of Mga to PspeB could not be demonstrated. Thus, Mga is able to both directly and indirectly regulate genes shown to be important for virulence and the metabolic homeostasis of GAS.

Mga is sufficient to activate transcription in vitro of sof-sfbX and other Mga-regulated virulence genes in the group A Streptococcus

The group A streptococcus (GAS), or Streptococcus pyogenes, is a strict human pathogen of medical significance, causing infections ranging from pharyngitis (strep throat) to necrotizing fasciitis (flesh-eating disease). Several virulence genes that encode factors important for colonization, internalization, and immune evasion are under the control of the multiple gene regulator of the GAS, or Mga. Mga functions as a DNA-binding protein that interacts with sites both proximal (Pemm and PscpA) and distal (PsclA) to the start of transcription for the genes that it regulates. The genes encoding serum opacity factor, sof, and a novel fibronectin-binding protein, sfbX, are cotranscribed and represent two uncharacterized Mga-regulated virulence genes in the GAS. Analysis of the promoter region of sof-sfbX identified a putative Mga-binding site 278 bp upstream of the regulated start of transcription as determined by primer extension. Electrophoretic mobility shift assays demonstrated that Mga is able to bind specifically to the single distal site in a fashion similar to the previously characterized PsclA. In order to better understand the events that take place at this and other Mga-regulated promoters, an in vitro transcription assay was established. Using this assay, we showed that Mga is sufficient to activate transcription in vitro for Mga-regulated promoters containing both proximal (Pemm) and distal (PsclA and Psof-sfbX) binding sites. These results indicate that additional factors are not required for Mga-specific activation at diverse promoters in vitro, although they do not rule out the potential influence of other components on the Mga virulence regulon in vivo.

Domains required for transcriptional activation show conservation in the mga family of virulence gene regulators

Mga, or the multigene regulator of the group A streptococcus (GAS) (Streptococcus pyogenes), is a transcriptional regulator of virulence genes important for colonization and immune evasion. All serotypes of the GAS possess one of two divergent mga alleles (mga-1 or mga-2), and orthologues of Mga have also been identified in other pathogenic streptococci. To date, the only functional motifs established within Mga are two amino-terminal DNA-binding domains (HTH-3 and HTH-4). To uncover novel domains, a random mutagenesis screen using an M6 Mga (mga-1) was undertaken to find mutations leading to a defect in transcriptional activation of the Mga-regulated emm gene. In addition to mutations in the established DNA-binding domains, the screen also revealed mutations in a region conserved among several Mga orthologues. Alanine scanning helped resolve the boundaries of this conserved Mga domain (CMD-1) spanning from residues 10 to 15 of the protein, with the two flanking amino acid residues likely involved in protein stability. Transcriptional reporter analyses demonstrated the importance of CMD-1 for activation of Pemm and autoactivation of Pmga in the serotype M6 Mga. Mutational analyses showed that both CMD-1 and HTH-4 are also necessary for activation of the promoter target Pmrp in a divergent serotype M4 Mga (mga-2), suggesting a conserved functionality. However, in contrast to M6, the M4 Mga mutants did not show a defect in autoregulation. Mutation of similar conserved residues in the Mga-like regulator DmgB from S. dysgalactiae subsp. dysgalactiae showed that CMD-1 and HTH-4 are critical for transcriptional activation in this orthologue, implying that a common mechanism of virulence gene activation may exist for members of the Mga family of regulators.

Patterns of virulence gene expression differ between biofilm and tissue communities of Streptococcus pyogenes

The ability of Streptococcus pyogenes to form biofilm-like bacterial communities during infection of soft tissue has suggested that the capacity to produce biofilm may be important for pathogenesis. To examine this relationship, a panel of mutants was evaluated for their ability to form biofilm on abiotic surfaces in several assays. Several established virulence factors were crucial for biofilm formation, including the M protein, required for initial cell-surface interactions, and the hyaluronic acid capsule, required for subsequent maturation into a three-dimensional structure. Mutants lacking the transcription regulators Mga and CovR (CsrR) also failed to form biofilm. Comparison of transcriptional profiles revealed differential regulation of approximately 25% of the genome upon adaptation to biofilm. During infection of zebrafish, several virulence factors (notably cysteine protease and streptokinase) were regulated in a biofilm-like manner. However, the overall profile of virulence factor expression indicated that tissue communities have a pattern of gene expression different from biofilm. Taken together, these data show that while biofilm and tissue communities have many characteristics in common, that biofilm reproduces only a subset of the myriad cues used by tissue communities for regulation of virulence.

Identification of residues responsible for the defective virulence gene regulator Mga produced by a natural mutant of Streptococcus pyogenes

Mga is a transcriptional regulator in the pathogen Streptococcus pyogenes that positively activates several important virulence genes involved in colonization and immune evasion in the human host. A naturally occurring mutant of Mga that is defective in its ability to activate transcription has been identified in the serotype M50 strain B514-Sm. Sequence alignment of the defective M50 Mga with the fully functional Mga from serotypes M4 and M49 revealed only three amino acid changes that might result in a defective protein. Electrophoretic mobility shift assays using purified M50 and M4 maltose binding protein-Mga found that both exhibited DNA-binding activity towards regulated promoters. Thus, the significance of each residue for the functionality of M50 Mga was explored through introduction of "gain-of-function" mutations based on M4 Mga. Transcriptional studies of the mutant alleles under both constitutive (PrpsL) and autoactivated (Pmga4) promoters illustrated that an arginine-to-methionine change at position 461 of M50 Mga protein fully restored activation of downstream genes. Western blot analyses of steady-state Mga levels suggest that the M461 residue may play a role in overall conformation and protein stability of Mga. However, despite the conservation of the M461 protein among all other Mga proteins, it does not appear to be necessary for activity in a divergent M6 Mga. These studies highlight the potential differences that exist between divergent Mga proteins in this important human pathogen.

MsmR, a specific positive regulator of the Streptococcus pyogenes FCT pathogenicity region and cytolysin-mediated translocation system genes

As a prerequisite for colonization or causing local infections, Streptococcus pyogenes (group A streptococci, GAS) need to specifically adhere to eukaryotic cell surfaces. Predominantly responsible adhesin genes are contained in a genotype-specific pattern within the FCT region of the GAS genome. In this study, MsmR, belonging to AraC/XylS type transcriptional regulators, was identified in the FCT region as a positive regulator of the major fibronectin-binding adhesin protein F2 in a serotype M49 strain. Compared with the wild-type strain, the msmR mutant showed reduced binding to immobilized fibronectin and decreased adherence to and internalization into human pharyngeal epithelial cells. These results suggested that altered levels of fibronectin-binding proteins in the mutant affect eukaryotic cell attachment and internalization. Complete transcriptome and reporter fusion assay data revealed that MsmR positively regulates FCT region genes including Nra and cytolysin-mediated translocation system genes. Consistent with the genetic data, the mutant showed attenuated streptolysin O activity and eukaryotic cell cytotoxity. Direct binding of recombinant MsmR to nga, nra/cpa and prtF2 promoter regions was confirmed by EMSA assays. As prior analysis demonstrated the Nra regulator negatively affects gene expression from the FCT region, MsmR and Nra appear to adversely control crucial virulence factor expression in GAS and thus contribute to a fine-tuned balance between local destructive process and metastatic spreading of the bacteria.

Transcriptional activation of sclA by Mga requires a distal binding site in Streptococcus pyogenes

Streptococcus pyogenes (the group A streptococcus [GAS]) is a medically significant pathogen of humans, causing a range of diseases from pharyngitis to necrotizing fasciitis. Several important GAS virulence genes are under the control of a pleiotropic regulator called Mga, or the multiple gene regulator of GAS, including the gene encoding the streptococcal collagen-like protein, or sclA. Analysis of the genome sequence upstream of sclA revealed two potential Mga-binding sites with homology to the published Mga-binding element, which were called PsclA-I (distal) and PsclA-II (proximal) based on their location relative to a predicted start of transcription. Primer extension was used to confirm that the Mga-dependent transcriptional start site for sclA was located adjacent to the proximal PsclA-II binding site. By using overlapping PsclA promoter probes and purified Mga-His fusion protein, it was shown by electrophoretic mobility shift assays that, unlike other Mga-regulated promoters, Mga binds only to a distal DNA-binding site (PsclA-I). Binding of Mga to PsclA-I could be competed with cold probes corresponding to known Mga-regulated promoters (Pemm, PscpA, and Pmga) but not with a nonspecific probe or the proximal PsclA-II fragment. With the use of a plasmid-based green fluorescent protein transcriptional reporter system, the full-length PsclA was not sufficient to reproduce normal Mga-regulated activation. However, studies using a single-copy gusA transcriptional reporter system integrated at the native sclA chromosomal locus clearly demonstrated that the distal PsclA-I binding site is required for Mga regulation. Therefore, PsclA represents a new class of Mga-regulated promoters that requires a single distal binding site for activation.

amrA encodes a putative membrane protein necessary for maximal exponential phase expression of the Mga virulence regulon in Streptococcus pyogenes

The transcriptional regulator Mga activates a regulon of virulence genes important for colonization and immune evasion in GAS. Using transposon mutagenesis of a serotype M6 group A streptococcus (GAS) reporter strain KSM148, we have identified an open reading frame (ORF) designated amrA that is required for maximal activation of the Mga regulon during exponential phase. A deletion in amrA, but not in the downstream transcriptionally linked ORF Spy0798, was able to reproduce the phenotype seen in the transposon mutants. Northern analysis for mga and emm transcripts, as well as Western analysis of Mga, confirmed a reduction in mga expression leading to a decrease in transcription of the Mga-regulated emm in the amrA deletion and transposon mutants. Furthermore, both the amrA deletion mutant and an original transposon mutant could be complemented using amrA expressed from a nisin-inducible expression system. As amrA is strongly conserved across the sequenced streptococcal M types, and inactivation of amrA in an M3 serotype also resulted in reduction of emm transcripts, the role of amrA does not appear to be serotype specific. Although the specific function of AmrA is unknown, its putative membrane localization and homology to transporters involved in cell wall synthesis suggest a link between growth and virulence gene expression in GAS.

Phase and antigenic variation in bacteria

Phase and antigenic variation result in a heterogenic phenotype of a clonal bacterial population, in which individual cells either express the phase-variable protein(s) or not, or express one of multiple antigenic forms of the protein, respectively. This form of regulation has been identified mainly, but by no means exclusively, for a wide variety of surface structures in animal pathogens and is implicated as a virulence strategy. This review provides an overview of the many bacterial proteins and structures that are under the control of phase or antigenic variation. The context is mainly within the role of the proteins and variation for pathogenesis, which reflects the main body of literature. The occurrence of phase variation in expression of genes not readily recognizable as virulence factors is highlighted as well, to illustrate that our current knowledge is incomplete. From recent genome sequence analysis, it has become clear that phase variation may be more widespread than is currently recognized, and a brief discussion is included to show how genome sequence analysis can provide novel information, as well as its limitations. The current state of knowledge of the molecular mechanisms leading to phase variation and antigenic variation are reviewed, and the way in which these mechanisms form part of the general regulatory network of the cell is addressed. Arguments both for and against a role of phase and antigenic variation in immune evasion are presented and put into new perspective by distinguishing between a role in bacterial persistence in a host and a role in facilitating evasion of cross-immunity. Finally, examples are presented to illustrate that phase-variable gene expression should be taken into account in the development of diagnostic assays and in the interpretation of experimental results and epidemiological studies.

Virulence factor regulation and regulatory networks in Streptococcus pyogenes and their impact on pathogen-host interactions

Streptococcus pyogenes (group A streptococcus, GAS) is a very important human pathogen with remarkable adaptation capabilities. Survival within the harsh host surroundings requires sensing potential on the bacterial side, which leads in particular to coordinately regulated virulence factor expression. GAS 'stand-alone' response regulators (RRs) and two-component signal transduction systems (TCSs) link the signals from the host environment with adaptive responses of the bacterial cell. Numerous putative regulatory systems emerged from GAS genome sequences. Only three RRs [Mga, RofA-like protein (RALP) and Rgg/RopB] and three TCSs (CsrRS/CovRS, FasBCAX and Ihk/Irr) have been studied in some detail with respect to their growth-phase-dependent activity and their influence on GAS-host cell interaction. In particular, the Mga-, RALP- and Rgg/RopB-regulated pathways display interconnected activities that appear to influence GAS colonization, persistence and spreading mechanisms, in a growth-phase-related fashion. Here, we have summarized our current knowledge about these RRs and TCSs to highlight the questions that should be addressed in future research on GAS pathogenicity.

Bacillus anthracis genetics and virulence gene regulation

The Bacillus anthracis genome consists of an approximately 5.3-Mb chromosome and two plasmids, pXO1 (182 kb) and pXO2 (96 kb). Genetic analysis has focused primarily on the structural genes for the anthrax toxin proteins, pagA, lef, and cya, the biosynthetic genes for capsule synthesis, capB, capC, and capA, and a gene associated with depolymerization of capsule, dep. The three toxin genes are located at distinct loci on pXO1, while the cap and dep genes are arranged in an apparent operon on pXO2. Additional genes that may play a role in B. anthracis virulence include the germination operon gerX and the general stress transcription factor sigB. Host-related signals affecting transcription of the toxin and capsule genes include temperature (37 degrees C) and bicarbonate/CO2. The B. anthracis plasmids carry two regulatory genes that share little sequence similarity with regulators in other bacteria. The pXO1-encoded gene atxA positively controls expression of the toxin and capsule genes, and has been implicated in control of other genes of unknown function. atxA mutants are avirulent in mice, and mice infected with atxA-null strains show a decreased immunological response to the toxin proteins. The pXO2-encoded regulator, acpA, shares sequence similarity with atxA. Yet acpA function appears to be restricted to positive control of capsule gene expression. The chromosomal gene abrB, a homologue of a well-characterized B. subtilis transition state regulator, controls growth phase-specific transcription of the toxin genes. Genetic manipulation of B. anthracis can be achieved by using natural means of DNA transfer and by electroporation of recombinant DNAs into B. anthracis. Genetic exchange can occur between B. anthracis strains and between B. anthracis and closely-related species. Although pXO1 and pXO2 are not self-transmissible, these plasmids and others can be transferred by conjugative plasmids originating in B. thuringiensis. Generalized transducing phage that permit inter-species transfer of chromosomal and plasmid DNA have also been described.

Molecular characterization of a novel fibronectin-binding protein of Streptococcus pyogenes strains isolated from toxic shock-like syndrome patients

Group A Streptococcus pyogenes has surface-located fibronectin (Fn)-binding proteins known to be a major virulence factor, which adheres to and invades host cells. We present a novel Fn-binding protein of group A streptococcus serotype M3 and M18 strains isolated from patients with toxic shock-like syndrome (TSLS). By searching the whole genome sequence of an M3 strain from a TSLS patient, an open reading frame was found among the putative surface proteins. It possessed an LPXTG motif and Fn-binding repeat domains in the C-terminal region and was designated as FbaB (Fn-binding protein of group A streptococci type B). The fbaB gene was found in all M3 and M18 strains examined, although not in other M serotypes. Furthermore, FbaB protein was expressed on the cell surface of TSLS strains but not on non-TSLS ones. Enzyme-linked immunosorbent assay and ligand blotting revealed that recombinant FbaB exhibits a strong Fn-binding ability. An FbaB-deficient mutant strain showed 6-fold lower adhesion and invasion efficiencies to HEp-2 cells than the wild type. Moreover, mortality was decreased in mice infected with the mutant strain in comparison to the wild type. These data suggest that FbaB is etiologically involved in the development of invasive streptococcal diseases.